In a typical cellular radio system also referred to as wireless communication network, wireless terminals, also referred to as user equipment, UEs, mobile terminals, and/or mobile stations, communicate via a radio access network (RAN) with one or more core networks, which provide access to data networks, such as the Internet, and/or the public-switched telecommunications network (PSTN). The RAN covers a geographical area that is divided into cell areas, with each cell area being served by a radio base station, also referred to as a base station, a RAN node, a “NodeB”, and/or enhanced NodeB “eNodeB”. A cell area is a geographical area where radio coverage is provided by the base station equipment at a base station site. The base stations communicate through radio communication channels with wireless terminals within range of the base stations.
Cellular communications system operators have begun offering mobile broadband data services based on, for example, Wideband Code-Division Multiple Access (WCDMA), High-Speed Packet Access (HSPA), and Long Term Evolution (LTE) wireless technologies. Moreover, fueled by the introduction of new devices designed for data applications, end user performance requirements are steadily increasing. The increased adoption of mobile broadband has resulted in significant growth in traffic handled by high-speed wireless data networks. Accordingly, techniques that allow cellular operators to manage networks more efficiently are desired. Techniques to improve downlink performance may include 4-branch Multiple Input Multiple Output (MIMO), multiflow communication, multi carrier deployment, etc. Since spectral efficiencies per link may be approaching theoretical limits, next steps may include improving spectral efficiencies per unit area. Further efficiencies for wireless networks may be achieved, for example, by changing a topology of traditional networks to provide increased uniformity of user experiences throughout a cell. Currently, so-called heterogeneous networks are being developed for 3GPP as discussed, for example, in: RP-121436, Study on UMTS Heterogeneous Networks, TSG RAN Meeting #57, Chicago, USA, 4-7 Sep. 2012; R1-124512, Initial considerations on Heterogeneous Networks for UMTS, Ericsson, ST-Ericsson, 3GPP TSG RAN WG1 Meeting #70bis, San Diego, Calif., USA, 8-12 Oct. 2012; and R1-124513, Heterogeneous Network Deployment Scenarios, Ericsson, ST-Ericsson, 3GPP TSG-RAN WG1 #70bis, San Diego, Calif., USA, 8-12 Oct. 2012.
A homogeneous network is a network of base stations, also referred to as NodeB's, enhanced NodeB's, or eNBs, in a planned layout, providing communications services for a collection of mobile terminals in which all base stations may have similar transmit power levels, antenna patterns, receiver noise floors, and/or backhaul connectivity to the data network. Moreover, all base stations in a homogeneous network may offer unrestricted access to mobile terminals in the network, and each base station may serve roughly a same number of mobile terminals. Current cellular wireless communications systems in this category may include, for example, Global System for Mobile communication (GSM), WCDMA, High Speed Downlink Packet Access (HSDPA), LTE, Worldwide Interoperability for Microwave Access (WiMAX), etc. In a heterogeneous network, low power base stations, also referred to as low power nodes (LPN), micro nodes, pico nodes, femto nodes, relay nodes, remote radio unit (RRU) nodes, small cells, RRUs, etc. may be deployed along with or as an overlay to planned and/or regularly placed macro base stations. A macro base station (MBS) may thus provide service over a relatively large macro cell area and each LPN may provide service for a respective relatively small LPN cell area within the relatively large macro cell area. Power transmitted by an LPN, e.g., 2 Watts, may be relatively small compared to power transmitted by a macro base station, e.g., 40 Watts for a typical MBS. An LPN may be deployed, for example, to reduce/eliminate a coverage hole(s) in the coverage provided by the MBS, and/or to off-load traffic from macro base stations, e.g., to increase capacity in a high traffic location, also referred to as a hot-spot. Due to the lower transmit power and smaller physical size, an LPN may offer greater flexibility for site acquisition.
In initial discussions among members of the 3rd-Generation Partnership Project (3GPP) regarding the development of Release 12 specifications for LTE, one of the proposed items for study is the possibility of simultaneously serving a mobile terminal from more than one eNB. In the disclosure that follows, this is called “dual connectivity.” The control plane procedures of LTE have to be updated in order to support this dual connectivity.
Dual connectivity is a feature defined from the mobile terminal perspective, whereby the mobile terminal may simultaneously receive and transmit to at least two different network points. Dual connectivity is defined for the case when the aggregated network points operate on the same or separate/different frequencies. Each network point that the mobile terminal is aggregating may define a stand-alone cell or it may not define a stand-alone cell. It is further foreseen that from the mobile terminal perspective, the mobile terminal may apply some form of Time Division Multiplexing (TDM) scheme between the different network points that the mobile terminal is aggregating in some scenarios, e.g. a scenario where the mobile terminal has less number of transmitter (TX) or receiver (RX) chains compared to the number of nodes it is connected to. This implies that the communication on the physical layer to and from the different aggregated network points may not be truly simultaneous in some scenarios. Thus, rather than purely simultaneous communications, dual connectivity may be regarded as providing support for contemporaneous communications with multiple independent network points, where “contemporaneous” should be understood as referring to events or things occurring or existing during the same period of time, where the periods of time relevant here are time periods relevant to wireless communications, i.e., on the scale of transmission time intervals, communications frame times, round-trip times, etc. The independence of the network points are understood as unrelated time sequences between nodes, e.g. unsynchronized subframe and frame time boundaries, etc.
Dual connectivity as a feature bears many similarities with carrier aggregation and coordinated multipoint (CoMP) communication, which are also technology areas undergoing rapid development in 3GPP and elsewhere. A main differentiating factor between dual connectivity and these other technologies is that dual connectivity does not require stringent synchronization between the wireless network access points and accommodates relaxed backhaul communication requirements. Besides, all the control plane processes, e.g. Automatic Repeat-reQuest (ARQ) signalling, radio link monitoring, signalling, etc. are completely independent with respect to the wireless network access points. For example, in dual connectivity mode, there will be one primary cell in both wireless network access points, thus the control plane will be terminated in separate wireless network access points. In this way, not only the data place, the control plane is also independent in dual connectivity. This is in contrast to carrier aggregation and CoMP, where tight synchronization and a low-delay backhaul are assumed between connected wireless network access points. The mobile terminal with dual connectivity may in some cases not have enough power to transmit to both wireless network access points leading to a reduced performance of the wireless communication network.